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#1
04-08-2010, 12:16 PM
 drhess Registered User Join Date: May 2002 Location: oklahoma Posts: 1,360
how many gallons of water equals 1 inch/acre of rain?

Ingnoring distribution problems, how many gallons of water do you need to "pour" onto an acre of land to equal one inch of rain fall? Feel free to answer in metrics.
#2
04-08-2010, 12:19 PM
 Apex Rogers Guest Join Date: Feb 2003 Location: San Francisco, CA Posts: 519
1 acre = 43560 sq ft. * 1/12 ft = 3630 cu. ft.

Using the google calculator: 3630 (cu feet) = 27 154.2857 US gallons
#3
04-08-2010, 12:53 PM
 drhess Registered User Join Date: May 2002 Location: oklahoma Posts: 1,360
So about 28,000 gallons per acre for an inch of "rain." Thanks.

Ok, next dumb question: how many inches of rain does a region need for land to be arable (assuming soil quality is ok)? I see the average worldwide rainfall is 39 inches/year. Over what range of rainfall would you need to grow a vareity of staple crops (garden or local market crops) or at least some crops that are on the less "thirsty" end of the scale? Or does soil quality make a huge difference in this calculation?
#4
04-08-2010, 01:08 PM
 lazybratsche Guest Join Date: Feb 2006 Posts: 3,705
Quote:
 Originally Posted by drhess Ok, next dumb question: how many inches of rain does a region need for land to be arable (assuming soil quality is ok)? I see the average worldwide rainfall is 39 inches/year. Over what range of rainfall would you need to grow a vareity of staple crops (garden or local market crops) or at least some crops that are on the less "thirsty" end of the scale? Or does soil quality make a huge difference in this calculation?
There's a huge range of variation here. There are some crops that will provide a modest yield in dry climates. And you can grow just about anything in the desert if you water it enough. Most farms in the US use irrigation (some much more than others). In many cases, the irrigation isn't necessary but it helps improve crop yields.

For one upper bound for "enough" water for current agricultural practices, I know that farmers in northern Ohio get enough rainfall that most don't bother to irrigate. There, the annual rainfall is approximately 40 inches/year.
#5
04-08-2010, 01:22 PM
 naita Guest Join Date: Jun 2002 Location: Norway Posts: 5,140
Quote:
 Originally Posted by Apex Rogers 1 acre = 43560 sq ft. * 1/12 ft = 3630 cu. ft. Using the google calculator: 3630 (cu feet) = 27 154.2857 US gallons
Abuse of the equals sign, B-
#6
04-08-2010, 01:52 PM
 CurtC Guest Join Date: Dec 1999 Location: Texas Posts: 6,482
Quote:
 Originally Posted by Apex Rogers 1 acre = 43560 sq ft. * 1/12 ft = 3630 cu. ft. Using the google calculator: 3630 (cu feet) = 27 154.2857 US gallons
The Google calculator can do it in one step:

In case the URL gets truncated, do a Google search on "1/12 acre-feet in gallons".
#7
04-08-2010, 02:03 PM
 dracoi Guest Join Date: Dec 2008 Posts: 8,867
Quote:
 Originally Posted by drhess Ok, next dumb question: how many inches of rain does a region need for land to be arable (assuming soil quality is ok)? I see the average worldwide rainfall is 39 inches/year. Over what range of rainfall would you need to grow a vareity of staple crops (garden or local market crops) or at least some crops that are on the less "thirsty" end of the scale? Or does soil quality make a huge difference in this calculation?
You also have to factor in when the rain falls.

In the Pacific Northwest, we get some rain year-round, but the bulk of it comes during the winters. Not much help during a summer growing season unless you have the technology to store the water and use irrigation. (In fact, we're pretty reliant on melting snow in the Cascades for our summer water. Low winter snowfall means droughts in the summer.)

Parts of Arizona actually get significant amounts of rainfall, but it comes during their "monsoon season" as short, heavy deluges. It can't all soak in and runs off with impressive flash floods, etc. So this would also not be very helpful for crops.
#8
04-08-2010, 02:04 PM
 pulykamell Charter Member Join Date: May 2000 Location: SW Side, Chicago Posts: 43,102
Quote:
 Originally Posted by CurtC The Google calculator can do it in one step: http://www.google.com/search?q=1/12+...eet+in+gallons In case the URL gets truncated, do a Google search on "1/12 acre-feet in gallons".
Or even "1 acre inch in gallons"
#9
04-08-2010, 02:05 PM
 Indistinguishable Guest Join Date: Apr 2007 Posts: 10,525
Quote:
 Originally Posted by naita Abuse of the equals sign, B-
How so?

ETA: Nevermind, I was reading the first period as a sentence break, whereas it's probably just part of the abbreviation "ft.", in which case, I agree, abuse of the equals sign

Last edited by Indistinguishable; 04-08-2010 at 02:06 PM.
#10
04-08-2010, 04:08 PM
 drhess Registered User Join Date: May 2002 Location: oklahoma Posts: 1,360
Quote:
 Originally Posted by naita Abuse of the equals sign, B-
What? Communicate clearly, D-.
#11
04-08-2010, 04:26 PM
 friedo Guest Join Date: May 2000 Location: Brooklyn Posts: 23,653
Quote:
 Originally Posted by Indistinguishable How so? ETA: Nevermind, I was reading the first period as a sentence break, whereas it's probably just part of the abbreviation "ft.", in which case, I agree, abuse of the equals sign
What is abusive about the equals sign there? One acre is indeed 43,560 sq. ft.
#12
04-08-2010, 04:42 PM
 Disheavel Guest Join Date: Nov 2006 Location: Seattle Posts: 1,335
Quote:
 Originally Posted by friedo What is abusive about the equals sign there? One acre is indeed 43,560 sq. ft.
but one acre does not equal 43,560 sq. ft. times 1/12 ft.
#13
04-08-2010, 04:43 PM
 penultima thule Guest Join Date: Apr 2009 Location: Sydney, Australia Posts: 2,331
Quote:
 Originally Posted by drhess how many inches of rain does a region need for land to be arable (assuming soil quality is ok)? I see the average worldwide rainfall is 39 inches/year.
The demarcation between arable land and grazing land is known as
Goyder's_Line, which broadly follows the 10" isohyet in Australia.

That is a winter rainfall pattern for winter cereals.

The question is of reliability and distribution, not amount.
#14
04-08-2010, 04:49 PM
 friedo Guest Join Date: May 2000 Location: Brooklyn Posts: 23,653
Quote:
 Originally Posted by Disheavel but one acre does not equal 43,560 sq. ft. times 1/12 ft.
Well, then that's just incorrect operator precedence, then, not abuse of the equals sign specifically. You all get an F.
#15
04-08-2010, 06:42 PM
 Indistinguishable Guest Join Date: Apr 2007 Posts: 10,525
Well, I guess you could say "(1 acre = 43560 sq. ft.) * 1/12 ft = 3630 cu. ft." reads acceptably, and therefore, since parenthesization makes things acceptable, it's just a precedence issue, but it's not really as though = is an operation returning a value which we then multiply by 1/12 ft to get 3630 cu. ft. The parenthesized version is still using some notational "abuse"; the only reason it reads a little better is because that particular kind of notational abuse is more standardly employed. At any rate, I'm not inclined to call it a precedence issue. But, whatever. I'm going to stop this nitpicking hijack before it gets out of hand.

Last edited by Indistinguishable; 04-08-2010 at 06:43 PM.
#16
04-08-2010, 06:51 PM
 NinetyWt Registered User Join Date: May 2002 Location: Garden Spot of the South Posts: 9,135
Quote:
 Originally Posted by drhess Ingnoring distribution problems, how many gallons of water do you need to "pour" onto an acre of land to equal one inch of rain fall? Feel free to answer in metrics.
Not all of the rainfall will go into uptake by the plants. The amount that runs off rather than soaking in is partially a factor of the type of soil the plants are growing in, the slope of the land, and any evaporation that is taking place.
#17
04-08-2010, 07:50 PM
 Duckster Charter Member Join Date: Aug 2001 Posts: 14,527
Quote:
 Originally Posted by penultima thule That is a winter rainfall pattern for winter cereals.
So this is when you switch from Uncle Toby's to Weetbix?
#18
04-08-2010, 08:03 PM
 Apex Rogers Guest Join Date: Feb 2003 Location: San Francisco, CA Posts: 519
Quote:
 Originally Posted by Indistinguishable Well, I guess you could say "(1 acre = 43560 sq. ft.) * 1/12 ft = 3630 cu. ft." reads acceptably, and therefore, since parenthesization makes things acceptable, it's just a precedence issue, but it's not really as though = is an operation returning a value which we then multiply by 1/12 ft to get 3630 cu. ft. The parenthesized version is still using some notational "abuse"; the only reason it reads a little better is because that particular kind of notational abuse is more standardly employed. At any rate, I'm not inclined to call it a precedence issue. But, whatever. I'm going to stop this nitpicking hijack before it gets out of hand.
I considered splitting all of my operations to separate lines to avoid abusing the equal sign, but given the simple nature of the calculation, I thought it would be obvious what happened. I agree that it's a bit unclear, but come on, this isn't a math exam here!
#19
04-08-2010, 09:03 PM
 Indistinguishable Guest Join Date: Apr 2007 Posts: 10,525
Right, like I said, I have no desire to badger on with the nitpicking hijack. (The only reason I entered it in the first place was because I originally didn't see what naita was talking about and was thus curious.)
#20
04-08-2010, 09:07 PM
 UncleFred Guest Join Date: Jan 2008 Posts: 753
Quote:
 Originally Posted by drhess Ingnoring distribution problems, how many gallons of water do you need to "pour" onto an acre of land to equal one inch of rain fall? Feel free to answer in metrics.
Hey - I recently did a calculation like this. I wanted to know how many gallons of rain fell on our little 4.5 sq-mile town when we got a recent 8 inch drenching. (Answer - about 11 million gallons). It was important because we're having discussions about the capacity of our storm and sewage drain systems.
#21
04-08-2010, 09:49 PM
 NinetyWt Registered User Join Date: May 2002 Location: Garden Spot of the South Posts: 9,135
Quote:
 Originally Posted by UncleFred It was important because we're having discussions about the capacity of our storm and sewage drain systems.
Be aware that the peak flow which your storm drain system will be required to handle is related to the amount of rain which falls, but it isn't a direct correlation. In other words, if four inches of rain falls you won't get four inches of runoff; and the size, shape, and slope of the watershed dictate the resulting peak runoff. What you are looking for is cubic feet per second, which is a rate, not a volume.
#22
04-09-2010, 07:13 AM
 naita Guest Join Date: Jun 2002 Location: Norway Posts: 5,140
Quote:
 Originally Posted by Apex Rogers I considered splitting all of my operations to separate lines to avoid abusing the equal sign, but given the simple nature of the calculation, I thought it would be obvious what happened. I agree that it's a bit unclear, but come on, this isn't a math exam here!
Life's a math exam when you teach math.
#23
04-09-2010, 07:18 AM
 naita Guest Join Date: Jun 2002 Location: Norway Posts: 5,140
And it was only a silly nitpick for a giggle , but here's how to avoid separate lines and still keep things clear and correct:

1 acre * 1/12 ft = 43560 sq ft * 1/12 ft = 3630 cu. ft
#24
04-09-2010, 08:25 AM
 Gorsnak Charter Member Join Date: Jan 2003 Location: Saskaboom Posts: 8,882
This'd be a lot easier if y'all just converted to metric, did the volume calculation, and converted back.
#25
04-09-2010, 01:51 PM
 drhess Registered User Join Date: May 2002 Location: oklahoma Posts: 1,360
Well,this website (I thought I provided this earlier) does all this for us, so that's easier:

http://www.virtualsecrets.com/annual...alculator.html

I was curious about how much water is needed to irrigate arid land. The issue of runoff and the ability of the soil to take the water is all important. Also, not every bit of an acre needs water if the crops are spread out (like an orchard vs "drilled" soybeans). Overall, however, if you assume you need roughly 40 inches and there's a chance of severe drought, it sounds like half a million to 1 million gallons per year per acre puts you in the back of the envelope ball park (to mix metaphors wildly). Sadly that website doesn't give cubic meters of water per hectare which would be nice.

Anyway, at about 50 cents (US) per 1,000 gallons (also US) (which is about the cost that desalination plants seem to produce in several places), that's about US\$500 per acre per year for water (not including additional infrastructure). Assuming that's off, it seems it's may be up to \$1,000 per acre for 40 inches of "rain."

Bummer, that's pricey.
#26
04-09-2010, 05:47 PM
 penultima thule Guest Join Date: Apr 2009 Location: Sydney, Australia Posts: 2,331
1Mil gallon [US] = 3,785.4 cubic meter
1 acre = 0.404 hectare

At the Murray Irrigation Water Exchange, general security water is currently priced at AUD70/ML. This is the bottom of the annual cycle, peak prices have gone over AUD1,000/ML.
http://www.murrayirrigation.com.au/content.aspx?p=20021
#27
04-09-2010, 05:54 PM
 Apex Rogers Guest Join Date: Feb 2003 Location: San Francisco, CA Posts: 519
Quote:
 Originally Posted by naita And it was only a silly nitpick for a giggle , but here's how to avoid separate lines and still keep things clear and correct: 1 acre * 1/12 ft = 43560 sq ft * 1/12 ft = 3630 cu. ft
Point taken. And I knew the nitpick was meant in jest, but I couldn't let such a remark against my character go undefended.
#28
04-09-2010, 10:14 PM
 t-bonham@scc.net Guest Join Date: Mar 2003 Location: Minneapolis, MN Posts: 13,824
Quote:
 Originally Posted by drhess Ok, next dumb question: how many inches of rain does a region need for land to be arable (assuming soil quality is ok)?
Almost none, really.

Part of the Sonoran Desert in southern California became the Imperial Valley due to irrigation from the Colorado River. It is now a highly productive agricultural area, but still part of a desert area, with almost no actual rainfall.
#29
04-09-2010, 11:30 PM
 [cc] Guest Join Date: Jan 2005 Posts: 200

http://www.wolframalpha.com/input/?i...re+by+one+inch
#30
04-10-2010, 09:46 AM
 drhess Registered User Join Date: May 2002 Location: oklahoma Posts: 1,360
Quote:
 Originally Posted by t-bonham@scc.net Almost none, really. Part of the Sonoran Desert in southern California became the Imperial Valley due to irrigation from the Colorado River. It is now a highly productive agricultural area, but still part of a desert area, with almost no actual rainfall.
Yes. That is the point, how much water (in gallongs, piped) to replace how much rainfall (in inches, gratis).
#31
04-10-2010, 09:48 AM
 drhess Registered User Join Date: May 2002 Location: oklahoma Posts: 1,360
Quote:
 Originally Posted by [cc] Google calculator, p'shaw. http://www.wolframalpha.com/input/?i...re+by+one+inch
#32
04-10-2010, 09:54 AM
 Mangetout Charter Member Join Date: May 2001 Location: England Posts: 57,238
Did anyone take into account the curvature of the Earth? That acre-sized rain guage is wider at the top than the bottom.
#33
04-10-2010, 04:06 PM
 Bayesian Empirimancer Guest Join Date: Feb 2010 Location: 94% probability on Earth Posts: 131
Can I get the units in Hoppus feet?
#34
04-10-2010, 07:10 PM
 Enginerd Guest Join Date: Jun 2000 Posts: 5,734
Quote:
 Originally Posted by drhess I was curious about how much water is needed to irrigate arid land. The issue of runoff and the ability of the soil to take the water is all important. Also, not every bit of an acre needs water if the crops are spread out (like an orchard vs "drilled" soybeans). Overall, however, if you assume you need roughly 40 inches and there's a chance of severe drought, it sounds like half a million to 1 million gallons per year per acre puts you in the back of the envelope ball park (to mix metaphors wildly). Sadly that website doesn't give cubic meters of water per hectare which would be nice. Anyway, at about 50 cents (US) per 1,000 gallons (also US) (which is about the cost that desalination plants seem to produce in several places), that's about US\$500 per acre per year for water (not including additional infrastructure). Assuming that's off, it seems it's may be up to \$1,000 per acre for 40 inches of "rain." Bummer, that's pricey.
That sounds high to me. When you're irrigating in the western US, you're not buying ready-to-drink water from desalination plants - you're taking available natural water (either surface water or well water) and applying it directly to the fields by sprinklers or flood irrigation. Desalination is a thermodynamically expensive process, but you don't have to pay for that level of treatment if you're irrigating with fresh water.

Northern Nevada has very low humidity and gets about 7 inches (18 cm) of rain a year - most of that falls in the winter. Alfalfa crops here typically get around 4 acre feet of water a year. Only a fraction of that is actually taken up by the plant - much of it is returned to the environment through either percolation or surface runoff, and an enormous fraction is evaporated. Still, the total cost of irrigation is closer to \$100 per acre for surface diversions than \$1000 (per University of Nevada Cooperative Extension). That cost includes not only the cost of the water (\$25/acre-foot), but also the costs of maintenance for the ditches and equipment as well as fees for the irrigation management agency.
#35
04-18-2010, 02:02 PM
 drhess Registered User Join Date: May 2002 Location: oklahoma Posts: 1,360
Quote:
 Originally Posted by Enginerd That sounds high to me. When you're irrigating in the western US, you're not buying ready-to-drink water from desalination plants - you're taking available natural water (either surface water or well water) and applying it directly to the fields by sprinklers or flood irrigation. Desalination is a thermodynamically expensive process, but you don't have to pay for that level of treatment if you're irrigating with fresh water. Northern Nevada has very low humidity and gets about 7 inches (18 cm) of rain a year - most of that falls in the winter. Alfalfa crops here typically get around 4 acre feet of water a year. Only a fraction of that is actually taken up by the plant - much of it is returned to the environment through either percolation or surface runoff, and an enormous fraction is evaporated. Still, the total cost of irrigation is closer to \$100 per acre for surface diversions than \$1000 (per University of Nevada Cooperative Extension). That cost includes not only the cost of the water (\$25/acre-foot), but also the costs of maintenance for the ditches and equipment as well as fees for the irrigation management agency.
Isn't irrigation use the (partial) idea behind some desalination plants? I thought it was their use in the mideast?
#36
04-18-2010, 02:18 PM
 drhess Registered User Join Date: May 2002 Location: oklahoma Posts: 1,360
I guess this report answers my question, but the cost does seem to be falling and in some places subsidies may make sense:
http://www.fao.org/docrep/meeting/00...e.htm#P22_2151
#37
04-18-2010, 04:04 PM
 Oslo Ostragoth Guest Join Date: Feb 2004 Location: the Prairie Posts: 6,771
[back to the drawing board]

Last edited by Oslo Ostragoth; 04-18-2010 at 04:06 PM.
#38
04-18-2010, 04:11 PM
 Oslo Ostragoth Guest Join Date: Feb 2004 Location: the Prairie Posts: 6,771
Quote:
 Originally Posted by lazybratsche ... Most farms in the US use irrigation (some much more than others). In many cases, the irrigation isn't necessary but it helps improve crop yields....
Roughly 12% of USA cropland is irrigated. Cite That's a lot more than I would have thought.
#39
04-18-2010, 05:05 PM
 Triskadecamus Guest Join Date: Oct 1999 Location: I'm coming back, now. Posts: 7,419
27154.071529364 gallons

Last edited by Triskadecamus; 04-18-2010 at 05:05 PM.

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